The present invention relates to a neutral beam injector which is capable of recovering the energy of charged particles.
FIG. 1 is a schematic view of a neutral beam injector which heats a plasma inside a fusion reactor. A description will now be made with reference to a deuterium ion beam. Neutral deuterium molecules D.sub.2 are ionized in a discharge chamber 10. The deuterium ions D.sup.+ are accelerated to become high energy ions while passing through an accel-decel grid 12. The high energy deuterium ions D.sup.+ are guided to a neutralizing cell 14 which is charged with a neutral deuterium gas at a relatively high pressure. In the neutralizing cell 14, the high-energy deuterium ions D.sup.+ are subjected to charge-exchange reaction with the neutral deuterium molecules to become high-speed neutral particles D.degree.. These neutral deuterium particles D.degree. are injected in the plasma in a fusion reactor (not shown) to heat the plasma. The neutralization efficiency in the neutralizing cell 14 decreases with an increase in ion energy E as shown in FIG. 2. This data in FIG. 2 is disclosed in "Mixed Species in Intense Neutral Beams", K. H. Berkner, R. V. Pyle and J. W. Stearns, Proc. 1st. Topical Meeting on Tech. Cont. Nucl. Fusion, Proc. CONF-740402-PI, San Diego, Calif., 1974, vol. 1, pp. 392 to 400. The unneutralized ions are deviated from the flow of the neutral beam due to the self electric field and collide with the wall of the chamber, so that they consume their energy. In order to obtain neutral particles using high energy ions of 200 keV, it is important to effectively utilize the consumed energy and to prevent damage to the wall of the chamber or the like. For this purpose, a direct converter 16 is generally incorporated. The energy recovered by the direct converter 16 can be again utilized to obtain the high energy deuterium ions. An example of a direct converter of this type is disclosed in "Performance Analysis of In-Line Direct Converters for Neutral Beam Sources", D. J. Bender, W. L. Barr, and R. W. Moir, Proceedings of 6th Symposium on Engineering Problems of Fusion Research, San Diego Calif., 1975, pp. 184 to 190.
A neutral beam injector having a typical conventional direct converter will be described with reference to FIGS. 3 and 4. FIG. 3 is a schematic sectional view of a neutral beam injector 18, and FIG. 4 is a graph showing the potential distribution at each part of the neutral beam injector 18. The neutral beam injector 18 comprises a vacuum vessel 20, cryo pumps 22 for evacuating the vacuum vessel 20, a gas feed 24 for feeding the neutral molecules, a discharge chamber 10 having an electron-emissive filament 26 and a discharge electrode 28, an accel-decel grid 12, a neutralizing cell 14, a direct converter 16, and a drift tube 30 for guiding the high-speed neutral particles to the plasma. The direct converter 16 comprises of a collector 32 and a pair of electron suppressors 34a and 34b which are arranged in front of and behind the collector 32, respectively. The electron suppressor 34a prevents the electrons generated in the neutralizing cell 14 from flowing to the collector 32. The electron suppressor 34b prevents the electrons generated in the drift tube 30 from flowing to the collector 32. Cooling pipes 36 for preventing the temperature of the collector 32 and the suppressors 34a and 34b from rising are arranged around the collector 32 and the electron suppressors 34a and 34b. The collector 32 and the electron suppressors 34a and 34b are held inside the vessel 20 by means of insulators 38. The vessel 20, the neutralizing cell 14, and a first grid 12a are electrically connected each other. The first grid 12a, a second grid 12b, a third grid 12c and the discharge electrode 28 are insulated from each other by means of a insulator 40. The electron-emissive filament 26 and the discharge electrode 28 are also insulated by means of insulators 42.
The neutral beam injector 18 is electrically connected as shown in FIG. 3. The vacuum vessel 20, the neutralizing cell 14 connected to the vessel 20, and the first grid 12a in the accel-decel grid 12 are grounded. The filament 26 is connected to a filament heater power source B1, while the discharge electrode 28 is connected to a discharge power source B2. The third grid 12c is connected to the discharge power source B2 through a resistor R. The second grid 12b is set at a negative potential by a power source B3. Therefore, the second grid 12b functions as a decel grid for preventing back streaming of the electrons.
The discharge electrode 28 is set at a positive high potential Va, for example, 200 kV, by power sources B4 and B5. Therefore, the ions receive an energy corresponding to the potential Va. The electron suppressor 34a is set at a negative potential-Vsupl, for example, -60 kV, by a power source B6. The electron suppressor 34b is set at a negative potential-Vsup2, for example, -20 kV, by a power source B7. The collector 32 is connected to a node of the power sources B4 and B5. Therefore, the potential of the collector 32 is set at a positive potential Vcol, for example, 190 kV, by the power source B4. The potential distribution of the respective parts of the neutral beam injector 18 described above becomes as shown in FIG. 4. In order to avoid energy losses, it is necessary to reduce the difference .delta.V between the potential Vcol of the collector 32 and the potential Va of the high energy ions.
The conventional neutral beam injector 18 has drawbacks to be described below. Since the deflection of the ion beam is caused by the repelling force of the charges in the ion beam itself, the ion beam is not much deflected within a short travel distance. In order to improve the ion-collecting efficiency for a given current density and to reduce the thermal load density at the collector 32, it is necessary to arrange the collector 32 as far as possible from the neutral beam so that the ion beam may be sufficiently deflected. By the way, the ion beam current depends upon the space charge limited current. The space charge limited current, the location of the collector 32, and the collector potential Vcol have a predetermined relationship. Current density j of the deuterium ions D.sup.+ collected at the collector 32 may be expressed by the relation: EQU j.alpha.(Va.sup.3/4 +.delta.V.sup.3/4).sup.2 /d.sup.2 (1)
where d is the travel path of the ion beam between the suppressor 34a and the collector 32, and .delta.V is the difference between the initial potential Va of the high energy ions and the collector potential Vcol. From this relation, it is seen that the current density j is inversely proportional to the square of the distance d and is decreased with a decrease in potential difference .delta.V. In other words, the closer the collector 32 to the neutralizing cell 14 and the lower the collector potential Vcol, the larger the current density j. By the way, when the current density of the ion beam is set at a predetermined value so as to obtain a proper beam of neutral particles D.degree. and the collector potential Vcol is set at a predetermined value so as to obtain a predetermined ion-collecting efficiency, the location of the electrode is determined. Due to this limitation, the collector 32 is conventionally located at a position at which the ion beam is not sufficiently deflected. Then, the considerable ions are not collected by the collector 32 but leak through an opening which is formed at the center of each electrode and which is designed to permit passage of the neutral particles. Since the collector 32 is close to the neutralizing cell 14, the ion current density at the collector 32 is high. For this reason, the thermal load density of the collector cannot be decreased to a desirable value. This has degraded the ion-collecting efficiency of the neutral beam injector 18 and has made the design of the direct converter difficult.